Hydraulic control systems typically use fluid pressure pumps situated remotely with respect to powered actuators. Pressure distributor conduits extend between the high pressure side of the pump and the actuators. A fluid reservoir for hydraulic fluid is located on the inlet side of the pump.
Pressure distribution to the actuators is controlled by remote control valves that respond to control signals from a controller. Typical examples of a hydraulic control system of this kind may be seen by referring to U.S. Pat. No. 5,468,058 and U.S. Pat. No. 5,551,770. The '058 patent discloses an anti-lock brake system for an automotive vehicle, and the '770 patent discloses a yaw control system for an automotive vehicle. Control systems of this type are commonly used also for traction control purposes where wheel torque can be modulated in response to signals from wheel speed sensors that detect incipient wheel slip.
The pump in a control system for an automotive vehicle having anti-lock brake capabilities or yaw control capabilities receives a supply of low pressure hydraulic fluid from a reservoir. The pump distributes hydraulic fluid under high pressure to brake actuators. Anti-lock brake performance, traction control performance, and yaw control performance can be adversely affected during cold temperature operation by reason of the increased viscosity of the brake fluid. The ability of the system to apply braking torque to the vehicle wheels during operation of the system in certain control modes, --for example, when the driver is not applying braking pressure to the brake pedal, --may be degraded by cold operating temperatures because the increased viscosity of the brake fluid may restrict delivery of fluid to the brake actuators.
It is known design practice to compensate for the adverse affect of cold temperature by designing the hydraulic control system with large diameter pressure distribution conduits extending from the reservoir to the low pressure side of the pump, which reduces flow restriction in the flow path for the hydraulic fluid. It is known design practice also to provide for an increased pressure differential between the pump inlet and the hydraulic reservoir. This can be accomplished by using a pressurized reservoir or an auxiliary pre-charge pump or by applying master cylinder pressure with an electrically actuated booster. Pressurized accumulators also may be used in conjunction with a fluid pressure reservoir for storing reservoir fluid under high pressure to mitigate the effect of an increase in fluid viscosity as fluid temperature decreases.
U.S. Pat. No. 3,253,409 discloses a hydraulic brake booster system in which heat is generated by a fluid pump or by an independent heat source to increase oil temperature in the booster system. Such an arrangement would not be feasible, however, in an anti-lock brake system, a yaw control system or a traction control system where oil temperature on the inlet side of the control pump must be controlled to reduce viscosity.
U.S. Pat. No. 5,600,954 discloses a hydraulic control system for an agricultural tractor. The system has a pre-charge pump that communicates at its inlet side with an oil reservoir. A heating element in the reservoir maintains an elevated oil temperature. This is intended to control viscosity in the valve circuit on the outlet side of the control pumps. Such a system would not be feasible, however, in an anti-lock brake system, a yaw control system or a traction control system for an automotive vehicle because of a lack of viscosity control at the inlet side of the control pump.